(Adds further details from defence ministry)
ADEN, June 17 (Reuters) - Islamist militants linked to al Qaeda were abandoning one of the last cities where they still had a foothold after weeks of bombardments and airstrikes in a U.S.-backed government offensive, Yemeni officials said on Sunday.
Elsewhere in Yemen's south, a security official was killed in a bomb attack, showing the militants' ability to strike despite the loss of a territorial base they had held for over a year.
The Defence Ministry said large groups of fighters from the Islamist Ansar al-Sharia, which swears allegiance to al Qaeda in the Arabian Peninsula, had already withdrawn from Azzan in the southern province of Shabwa.
It borders Abyan province, the focus of the military campaign.
Ansar al-Sharia seized control of several cities in Abyan last year during a wave of protests against the three-decade rule of then leader Ali Abdullah Saleh, who redeployed some of his forces from the south to cope with the unrest.
Among the towns seized was Abyan's capital, Zinjibar, which the military said it had retaken last week.
The defence ministry said fleeing fighters had left behind large numbers of mines and explosives, and three people had been killed when a mine exploded in Zinjibar on Sunday.
The United States is increasingly concerned about the militant presence in Yemen and has supported the government forces with training, intelligence, drone strikes and more military aid. The Pentagon has declined to give details of the scale of the aid, however.
The U.S. State Department on Saturday congratulated the government on the success of the offensive and urged it to swiftly restore civilian authority, humanitarian relief and essential public services to southern cities.
Separately, a colonel who led security forces in the town of Rawkab, in Hadramout province, was killed in a bombing blamed on Islamist militants, a security official said.
Two soldiers were wounded in the attack outside the town's police station, the official said. (Reporting by Mohammed Mukhashaf,; Writing by Joseph Logan, Editing by Sophie Hares)